1. Field of the Invention
This invention relates to a ferritic stainless steel for use in various types of heat exchangers.
2. Description of the Related Art
In general, the structural components of the heat exchanger, particularly heat transfer plate and the like, should be made as thin as possible for ensuring a large heat transfer efficiency. In the heat transfer plate or the like of the heat exchanger, however, as the thickness of the plate becomes thinner, the degradation of properties and service life is caused due to the occurrence of scale peeling accompanied with repetitive oxidation, so that it is important that the components have a high resistance to oxidation.
Under the above circumstances, stainless steels such as SUS304, SUS430 and the like are generally used as a material for the conventional heat exchanger.
The above SUS304 (austenitic stainless steel) presently used as the material for the heat exchanger is large in the reduction of the thickness accompanied with the scale peeling and is difficult to be used at a higher temperature.
In general, it is said that the larger the heat transferring amount per unit time or heat conductivity, the better the heat exchanger. On the other hand, this has such a problem that the difference in thermal expansion based on the difference in temperature among parts of the heat exchanger in operation is promoted to cause thermal stress. Particularly, when the thermal stress exceeds the tensile strength of the material, the cracking is created to damage an important air tightness as a performance of the heat exchanger. Considering this point, therefore, it is said that the thermal expansion coefficient is favorably small as a material for the heat exchanger.
As mentioned above, it is demanded that the essentially conflicting properties are excellent as the material for the heat exchanger. In this point, it is actually said that SUS304 does not sufficiently respond to this demand.
On the contrary, it is considered that SUS430 (ferritic stainless steel) is used as a material for the heat exchanger. However, the SUS430 steel hardly causes the scale peeling accompanied with the repetitive oxidation but is low in high-temperature strength, so that it is apt to be deformed by thermal stress when a high-temperature fluid is present in the heat exchanger and hence the heat exchanging efficiency undesirable lowers. Furthermore, the soldering operation (e.g. copper soldering) is frequently conducted in the manufacture of the heat exchanger. In this case, slow cooling is carried out after the heating to not lower than 1100.degree. C., so that the material is sensitized to undesirably lower its corrosion resistance.
In order to prevent the degradation of the corrosion resistance due to the sensitization, therefore, there have hitherto been proposed ferritic stainless steels added with a stabilizing element such as SUS430LX and the like. However, SUS430LX tends to obstruct the solderability, so that it is required to improve upon this material because solderability has a very large influence upon heat exchanging efficiency.
If it is intended to heat exchange a combustion gas of a fluid containing sulfur, e.g. light oil as a fluid to be heat-exchanged, the corrosion resistance against sulfur-containing gas is naturally required, but the ferritic stainless steels such as SUS430 and SUS430LX are insufficient in such a corrosion resistance.
Furthermore, formability is mentioned as another important property of the material for the heat exchanger because the heat exchanger is generally manufactured by press forming. Therefore, material for the heat exchanger should be excellent in formability represented by a magnification of elongation, an Erichsen value or the like. This is also insufficient in the ferritic stainless steel such as SUS430 and the like.